Please Can I use a 60w MagSafe2 adapter on my 15-inch MacBook Pro with Retina display, which uses an Apple 85W MagSafe 2 Power Adapter?

I would not. It does not provide the needed wattage for charging the battery and could result in damage to the charger.

You can, with some caveats.

1. It will not charge the computer very well (or at all) while it is running

2. It will get hot as a consequence of running close to it's maximum output all the time, so put it somewhere with good airflow

3. It will always take longer to recharge the Mac, under all conditions.

4. It will probably fail earlier than it would have under normal circumstances.